1. Field of the invention
The present invention relates to a life time measuring apparatus for minority carriers of semiconductor wafers used for quality control of semiconductor wafers.
2. Description of the Prior Art
Along with an ultrahigh precision tendency in semiconductor devices represented by VLSI in recent years, severer quality control has come to be required for semiconductor wafers used in semiconductor devices. For such control, a noncontact type evaluation method is desirable which is free from contamination or damage to semiconductor wafers. As an example of a noncontact type evaluation method, a semiconductor characteristics measuring method using microwaves is well known (Japanese Patent Publication Sho-61-60576).
FIG. 8 is a typical view showing the schematic circuit configuration in an example of prior semiconductor characteristics measuring apparatus A0. FIG. 9 is a graph showing results from the semiconductor characteristics measuring apparatus A0.
As shown in FIG. 8, a prior semiconductor characteristics measuring apparatus consists of a specimen retention/transfer mechanism 47 (support base), an optical pulse generator 49 to irradiate optical pulses on the surface of the specimen 7 (semiconductor wafer) supported and transferred by the specimen retention/transfer mechanism 47, a gun oscillator 40 to generate microwaves to be radiated on the surface of the specimen 7, an impedance matching box 41 to regulate microwaves radiated from gun oscillator 40, a regulating mechanism 46 consisting of E-H tuners 42 and 44, magic T 3 and reflection-free terminal 43, waveguide 11 to radiate microwaves regulated by regulating mechanism 46 on the surface of the specimen 7, detector 45 to detect microwaves deflected on the surface of the specimen 7 by allowing it to pass again through waveguide 11 and regulating mechanism 46, and synchroscope 48 to display the change in the microwave detected by detector 45.
The measuring principle of said apparatus A0 is as follows:
In the specimen 7, majority carriers and minority carriers, free electron--positive hole pairs, are excited by optical pulse irradiated from optical pulse generator 49 the carrier concentration increases during light irradiation. On the other hand, between optical pulses where light irradiation is interrupted, excessive carriers (i.e., carrier concentration in excess of the carrier concentration at thermal equilibrium) gradually disappear due to recombination, decreasing carrier concentration. Such change in carrier concentration is remarkable on the minority carrier side. Since electric conductivity (specific resistance) of the specimen 7 changes according to change in carrier concentration, level change is caused in the microwave incident on the specimen 7. The microwave where level change is caused becomes a reflected wave to be transmitted to detector 45 through waveguide 11 and regulating mechanism 46. The reflected wave of the microwave detected here is displayed by synchroscope 48 as a damping curve. The life time of minority carriers of the specimen 7 can be measured from this damping curve.
In measuring the life time of minority carriers of the specimen 7 consisting of semiconductor wafers, it is necessary to carry out measurement without the effects of wafer thickness and wafer vibration in order to improve precision and to shorten measuring time. In other words, it is desired to conduct measurement without depending upon the distance between the opening end 6 of waveguide 11, a radiating end of microwaves, and the specimen 7.
FIG. 9 shows results of measuring the life time of minority carriers of silicon single crystal wafers (P and N types), specimen 7, with distance d between the opening end 6 of waveguide 11 and the specimen 7 as parameters. It is found that change in distance d has virtually no effect on measured results.
In said prior measuring apparatus A0, measurement not depending upon distance d between the opening end 6 of waveguide 11 and the specimen 7 may necessitate the following operations: removing unnecessary reflection from waveguide 11 having the opening end 6 with the use of two E-H tuners of regulating mechanism 46 and regulating the reflection quantity of microwaves from the specimen 7 variable depending upon a specific resistance of the specimen 7. In addition to what are respondable by said operations, however, there exist a multiple reflected wave between the opening end 6 and the specimen 7 and a reflected wave by a support base 47 for microwaves passing through the specimen 7. The quantity of these reflected waves vary depending upon specific resistance of the specimen 7. Therefore, in the specific resistance range of around twice that shown in FIG. 9, prior measuring system A0 is respondable, while for semiconductor wafers with a wide range of specific resistance, it is difficult to obtain constant measured results without depending upon distance d between the opening end 6 and the specimen 7.
In other words, specific resistance to measure semiconductor wafers usually covering a wide range from 1 to 100 .OMEGA.cm results in microwave amplitude reflectance from semiconductor wafers of a wide range of from 0.5 to 1.0. Since microwave reflectance variations due to carriers generated in semiconductor wafers when light is irradiated is extremely small, it is necessary to normally use tens of mW microwave power to radiate semiconductor wafers in order to catch signals to find the life time at high precision. On the other hand, unless input power to detector 45 is made below about 0.1 mW where the detection diode used for detector 45 has square-law characteristics, errors are caused in the measured values of life time due to nonlinear effect of the detection diode. In the microwave circuit configuration of said prior measuring apparatus A0, it was difficult to meet said detection diode input power range for semiconductor wafers with said wide-range specific resistance.
Furthermore, for measuring the intrinsic life time of semiconductor wafers at high precision, it is necessary to sufficiently reduce the quantity of minority carriers generated compared to originally existing majority carriers by reducing the quantity of light irradiated. However, reduced light quantity not only diminishes change in microwave reflectance in semiconductor wafers but also diminishes life time measuring signals obtained by detector 45. This made it difficult to measure by prior apparatus A0 semiconductor wafers with low specific resistance of below about 1 .OMEGA.cm.